Hi Everyone, my Fun 57000 miles genuine is playing me up, starts fine, but when you drive off then it is very jerky on acceleration  then it clears for awhile then when I slow a bit the it starts doing it again, seems like water in the gas  but clean gas just put in . fitted new plugs, leads seem ok What next ?   thanks,

Are all symptoms happenning only at low rpm?

Do you have access to a VAG-COM scanner to look for stored errors?

... if not, and a fault code read is definitely a good place to start, especially as the tools are quite cheap now.

 

Misfiring under load is most likely an ignition fault, not always, but most likely.

 

If you've got a spare plug lead that you know is good, or can borrow one that fits off another car, it's worth just swapping that in to each in turn and seeing if it helps. Same for the king lead

 

Obvious stuff, go along your ignition system and check for poor connections, rusty plugs and earths. Don't forget the ignition amplifier that lives under the ignition coil on most of these, and the wires that go to it.

 

To be fair, this little box of tricks is a pretty neglected part, they're not vastly expensive to buy (GSF do sell them) and a new one can tidy up the spark on a lot of older cars. I don't condone diagnosis by expenditure at all, but it's a good treat for your car. 

 

Then there's the scary stuff. Whip a plug out and see how it looks. Has it got oil on?

Thanks you for your advice, no sorry no VAG com,  so ill do it  the trial and error style, as suggested by Steves Truck, ...... I'm thinking ignition now.....by the way I've never heard of an ignition amplifier,  could this be the possible cause?  thanks  John.

Forgot to mention, no water in the oil, and the car runs perfect at higher speeds

... and the car runs perfect at higher speeds

To me this is enough to rule out ignition problems.

 

Remove and clean the throttle body. Clean the air temperature / manifold pressure combined sensor. Replace the gasket under the throttle body. Check for vacuum leaks.

Edited 18 January, 201610 yr by RicardoM

Will do, I will have a play about with it at the weekend, thanks for the advice, ill keep you informed,  John

Try changing coil pack and remove and clean throttle body just had same fault 9n my 16v and changed coil pack had code on vag com and she now runs sound

Try changing coil pack and remove and clean throttle body just had same fault 9n my 16v and changed coil pack had code on vag com and she now runs sound

Why would you put him buy a 'coil pack'? Just because you had the same symptoms on another type of engine? Based on his description of the symptoms, what evidence suggests the 'coil pack' is faulty?

If you have a multimeter you can do some primary and secondary resistance tests on the ignition coil to assess if its faulty, its a good shout though, you need more voltage to make a spark at lower engine loads. I have had these fail intermittently before.

but as you have stated it starts fine then plays up once the throttle is pressed, you can almost certainly dismiss an ignition fault or vacuum leak. i would start with cleaning and resetting the throttle body.

Edited 21 January, 201610 yr by TeflonTom

Sounds like the dizzy cap to me.. I would whip the cap off and check it for cracks before I did anything else. Rotor arms burn out too on these.

Get it in the dark with bonnet open and rev it you will see any stray sparks flying around.

Good dose of wd40 or similar in the cap and around the coil etc.

...you need more voltage to make a spark at lower engine loads.

Can you elaborate on this please?

Can you elaborate on this please?

its simple physics, more load is more heat, less voltage is needed to create a spark if the combustion chamber is hotter.

And at lower rpm the single coil has a longer charge period to create a stronger spark. The reason most petrol engines now use multiple coils.

I'm still going with dizzy cap or leads tbh..

At higher rpm the spark is weaker so you don't get the arcing that you do at lower rpm.

And at lower rpm the single coil has a longer charge period to create a stronger spark. The reason most petrol engines now use multiple coils.

I'm still going with dizzy cap or leads tbh..

At higher rpm the spark is weaker so you don't get the arcing that you do at lower rpm.

After reading the whole thread I'd agree.

Does anyone have a graph showing the secondary high voltage discharge peak over rpm for a single ignition coil, 4 cylinders, 4 stroke car? Even better, a live labscope recording?

 i would start with cleaning and resetting the throttle body.

 

Mr Bubble, I think I have a clean and reset one of these in the garage. Could do with a good home to go to ;)

Edited 24 January, 201610 yr by RainbowFire

The settings for he throttle body are in the ecu, not in the throttle body itself. It all centres round the adaptive running conditions which the ecu brings in when the throttle body gets dirty and fouled up with oil and sludge.

The other thing that's vital is that you don't unplug the throttle body whilst the ignition is turned on, I made this mistake 150 miles from home once, kangaroo'd itself all the way home until I could reset it with vcds.

Wow seems I've opened a can of worms here....Hi Rainbow , hope all's ok with you, thanks for the offer,  Ill try sparks first ,then throttle, ill try it one move at a time and evaluate it correctly, thanks guys for the suggestions............ Magic forum!

Ok throttle body taken off, cleaned replaced back on with new gasket..........taken for a spin.........no change.

I have seen somewhere that the ECU will reset when the battery is disconnected for 24 hours, is this correct? 

No. There is a specific procedure discussed many times in this section.

Did you clean the IAT / MAP combined sensor too as indicated ?

 

IAT = intake air temperature

MAP = manifold absolute pressure

 

If MAP sensor gets contaminated with engine oil, the engine stalls.

Edited 28 January, 201610 yr by RicardoM

I must admit that I did not, ill check it out tomorrow